Thats incorrect. The distance the shoes travel is dictated by the travel of the piston in the hydraulic cylinders. This travel is a function of fluid volume moved. The distance the shoes travel manifests itself as pedal travel.
The firmness of the pedal is a function of the compressibility of the working fluid in the sytem and its ability to apply force to the shoes once they are in contact with the drum. THAT is the definition of a firm pedal.
A 109 has more fluid volume in the front cylinders than the rear. To shift the shoes the same amount (Assumes well adjusted brakes) you need a larger volume in the bore closest to the pedal. The pressure of the fluid controls the flow rate at which the pistons move before contact and then the final force applied to the brake shoe once it is in contact.
The theory behind hydraulics is not that difficult. It would seem that 99% of all automotive manufacturers seem to follow the same set of rules about bigger brakes and the resulting bore size difference in the MC for the fronts vs rears. One exception is the 88 with its front andrear 10" drums. I agree with TAW on this one.